Sodium Silicate Cold-Bonded Chromite Pellets for the Ferrochromium Industry – Identifying a Suitable Process

Abstract

Chromite fines (defined as particles smaller than 6 mm) account for a significant fraction of mined South African chromite ore. These fines are pelletized to allow safe furnace smelting conditions. However, pelletization is an energy-and cost-intensive process that requires significant capital investment and has high operational costs as continuous curing temperatures of 1300-1500 °C are required. In this study, various cold-bonded pelletization processes were investigated to identify a process to prepare cold-bonded chromite pellets. A feasible process was identified and developed to produce mechanically strong chromite pellets using 3wt% sodium silicate-3 wt% SiO 2 as the binder and KOH as an alkaline activator. The binding mechanism is based on the combined effects of sodium silicate and geopolymerization. It was determined that pellets cured at 75 °C had compressive strengths of 2.41-3.12 kN. The mechanical strength of these cold-bonded pellets where similar to, or exceed that of pellets currently used in the FeCr industry.

An Overview of Currently Applied Ferrochrome Production Processes and Their Waste Management Practices

“…Figure 15 shows pellets with a compressive strength of 3.12 ± 0.10 kN were produced using a combination of 3 wt% sodium silicate-3 wt% SiO2, a 6 wt% moisture addition using a 4 M KOH solution, and a curing temperature of 75 °C. These pellets greatly exceeded the compressive strength of 1.54 ± 0.50 kN determined for pre-reduced pellets obtained from a South African FeCr producer [113]. Cold-bonded pelletization is typically not considered on a large scale as existing pellet curing infrastructure, i.e., pre-reduction and oxidative sintering, is well established and understood.…”

“…The addition of SiO 2 and the use of an alkaline activator likely promoted geopolymerization of the Al-and Si-oxides present in the pelletization material, which contributed to the mechanical strength of the cold-bonded pellets. Figure 15 shows pellets with a compressive strength of 3.12 ± 0.10 kN were produced using a combination of 3 wt% sodium silicate-3 wt% SiO 2 , a 6 wt% moisture addition using a 4 M KOH solution, and a curing temperature of 75 • C. These pellets greatly exceeded the compressive strength of 1.54 ± 0.50 kN determined for pre-reduced pellets obtained from a South African FeCr producer [113].…”

“…It is understood that the agglomerated chromite has to meet certain specifications before it can be safely smelted in an SAF. These specifications include the following: (i) the agglomerate must be mechanically strong as to prevent the reformation of fines in the upper part of the furnace bed; (ii) the binder addition must be kept to a minimum as to limit Cr-dilution, with additions up to 3 wt% generally regarded as acceptable; (iii) the binder should not contribute to the formation of environmentally adverse compounds within the SAF, specifically Cr(VI); (iv) the binder must not adversely affect the furnace chemistry (e.g., for a certain smelting regime, the basicity factor is typically predictable); and (v) the binder should ideally not contribute to the Si content of the produced FeCr [113].…”

“…A study by Du Preez et al (2020) investigated a variety of binders for the pelletization processes as well as a subsequent low-temperature curing method. The use of metasodium silicate, precipitated SiO 2 , and an alkaline activator was identified as an acceptable binder combination, and pellet curing at 75 • C was used to produce mechanically strong pellets [113]. The addition of SiO 2 and the use of an alkaline activator likely promoted geopolymerization of the Al-and Si-oxides present in the pelletization material, which contributed to the mechanical strength of the cold-bonded pellets.…”